Living is an essential part of life, evidenced by the history of housing development, and lock was invented as a way to secure safe living. Though there are kinds of lock to safeguard houses, a lot of defects exist, such as:
1. Key brough away by any prior room-mate will make the present occupant unable to have full protection from the existing lock and the prior room-mate may access without any difficulty. Therefore, replacement of lock set is necessary after leaving of any room-mate and it means a waste of time and money.
2. Generally a fixed barrel is used for each lock set and thus variation of unlocking keys is limited, and replacement of barrel is expensive. Lock set of such kind can be broken or opened by burglar easily.